Diesel engines and other lean-burn engines or power plants are operated at higher than stoichiometric air-to-fuel mass ratios for improved fuel economy. Such lean-burning engines produce a hot exhaust with a relatively high content of oxygen and nitrogen oxides (NOx). The temperature of the exhaust from a warmed-up diesel engine is typically in the range of 200° C. to 400° C. and has a representative composition, by volume, of about 10-17% oxygen, 3% carbon dioxide, 0.1% carbon monoxide, 180 ppm hydrocarbons, 235 ppm NOx and the balance nitrogen and water. These NOx gases, typically comprising nitric oxide (NO) and nitrogen dioxide (NO2), are difficult to reduce to nitrogen (N2) because of the high oxygen (O2) content in the hot exhaust stream.
U.S. Pat. Nos. 6,957,528 and 7,093,429, and U.S. Patent Application Publication 2006/0283175, each assigned to the same assignee as this invention, disclose methods of adding ozone and nonthermal plasma-reformed diesel fuel constituents to the exhaust stream flowing from a lean burn engine or power plant preparatory to selective catalytic reduction (SCR) of NOx. Ozone is added to the exhaust stream for oxidation of NO to NO2. And air/ozone plasma-generated, low molecular weight, oxygenated hydrocarbons and hydrocarbons from a fractionated portion of the diesel fuel hydrocarbon mixture are added to the exhaust as reductants for conversion of NO2 to N2 over a selective reduction catalyst. This process may be called diesel fuel/SCR. Cu/Y zeolite and Ag/alumina catalysts have been considered for use in SCR. However, there remains a need for improvement in the conversion of NOx in diesel exhaust and in the exhaust of other lean burn engines.